Fix GH-23056: missing handler name in session write warning

[lazerg]

When the session data is unchanged, lazy write calls the update timestamp handler, and the warning names ps_update_timestamp. That zval stays undefined for handlers that do not provide an updateTimestamp() method, so the name came out empty. In that case ps_update_timestamp_user() calls the write handler instead, so the warning should name the write handler.

This only shows on master because an empty session now encodes to an empty string rather than NULL, which lets the lazy write branch run where it previously did not.

Fixes GH-23056.

[lazerg]

The WINDOWS_X64_ZTS failure (ext/standard/tests/mail) is unrelated to this change. It fails the same way on master's own tip (d7911aa), so it looks like the Windows mail test environment is currently broken independent of this PR.